Radar level gauge (RLG) systems are in wide use for determining the filling level of a product contained in a tank. Radar level gauging is generally performed either by means of non-contact measurement, whereby electromagnetic signals are radiated towards the product contained in the tank, or by means of contact measurement, often referred to as guided wave radar (GWR), whereby electromagnetic signals are guided towards and into the product by a probe acting as a waveguide. The probe is generally arranged to extend vertically from the top towards the bottom of the tank. The probe may also be arranged in a measurement tube, a so-called chamber that is connected to the outer wall of the tank and is in fluid connection with the inside of the tank.
The transmitted electromagnetic signals are reflected at the surface of the product, and the reflected signals are received by a receiver or transceiver comprised in the radar level gauge system. Based on the transmitted and reflected signals, the distance to the surface of the product can be determined.
More particularly, the distance to the surface of the product is generally determined based on the time between transmission of an electromagnetic signal and reception of the reflection thereof in the interface between the atmosphere in the tank and the product contained therein. In order to determine the actual filling level of the product, the distance from a reference position to the surface is determined based on the above-mentioned time (the so-called time-of-flight) and the propagation velocity of the electromagnetic signals.
Most radar level gauge systems on the market today are either so-called pulsed radar level gauge systems that determine the distance to the surface of the product contained in the tank based on the difference in time between transmission of a pulse and reception of its reflection at the surface of the product, or systems that determine the distance to the surface based on the frequency difference between a transmitted frequency-modulated signal and its reflection at the surface. The latter types of systems are generally referred to as being of the FMCW (Frequency Modulated Continuous Wave) type.
For pulsed radar level gauge systems, time expansion techniques are generally used to resolve the time-of-flight.
Such pulsed radar level gauge systems may have a first oscillator for generating a transmission signal formed by pulses for transmission towards the surface of the product contained in the tank with a transmitted pulse repetition frequency ft, and a second oscillator for generating a reference signal formed by reference pulses with a reference pulse repetition frequency fref that differs from the transmitted pulse repetition frequency by a given frequency difference Δf. This frequency difference Δf is typically in the range of Hz or tens of Hz.
At the beginning of a measurement sweep, the transmission signal and the reference signal are usually synchronized to have the same phase. Due to the frequency difference Δf, the phase difference between the transmission signal and the reference signal will gradually increase during the measurement sweep. During the measurement sweep, the reflection signal resulting from reflection of the transmission signal at the surface of the product contained in the tank is being correlated with the reference signal, so that an output signal is only produced when a reflected pulse and a reference pulse occur at the same time. The time from the start of the measurement sweep to the occurrence of the output signal resulting from the correlation of the reflection signal and the reference signal is a measure of the phase difference between the transmission signal and the reflection signal, which is in turn a time expanded measure of the time-of-flight of the reflected pulses, from which the distance to the surface of the product contained in the tank can be determined.
The aforementioned synchronization of the transmission signal and the reference signal requires regulation to be within acceptable parameters. WO 01/02819 describes a frequency regulator for creating and synchronizing transmit and sample clocks. A controller feeds back a control output to the clock source, and this feedback is used to stabilize a first frequency separation between first and second clock frequencies, so that the radar gauge circuit generates a level output that is stabilized and also corrected as a function of the frequency separation.
However, current radar level gauge systems are still power hungry and it is hard to meet safety requirements regarding power usage and energy storage. Further, recent developments in the field of radar level gauging call for more stand alone functionality and systems being capable of operating in far away and remote sites where power limitations can be more severe, e.g. the systems cannot be supplied by the grid. Hence, there is a need to minimize the power used in radar level gauges, and further to provide a radar level gauge which is able to operate independently and provide a larger amount of functionality either on-site or off-site by providing more measurements at a lower total energy cost. Even further, there is a need to meet the above mentioned safety regulations limiting the amount of energy which is allowed to be stored due to inherent hazards such as the material in the storage tanks being highly flammable etc.